Along with the development of the electronics industry, which requests the electronic components having high performance and miniaturization, as for the sintered tantalum and/or niobium electrolytic capacitor must be having high capacitance (CV) in unit volume and in unit weight of the powder. This will need using high BET (Brurauer-Emmet-Teller) surface area tantalum and/or niobium powders which have fine primary particles to manufacture the capacitor anode.
The tantalum, niobium metal powders are usually produced by chemical reduction of a tantalum or niobium compound with a reducing agent or by hydrogenating the tantalum or niobium metal ingot and pulverizing the hydrogenated ingot. The tantalum, niobium metal powders prepared by the former process are porous basic agglomerates having high BET surface area comprised of many primary particles, and the later process generally produces powders with high purity but low surface area.
The parameters for characterizing the size of the metal particles include BET surface area (m2/g) measured by the liquid nitrogen adsorbed, Fisher sub-sieve particle size (FSSS/μm), powder particle size distribution measured with a laser particle size distribution analyzer, particle size and morphology measured with scanning electron microscopy and transmission electron microscopy. The BET surface area is associated with the particle size. The finer the primary particles are, the larger the BET surface area is, and generally the higher the capacitance of the powder has. The FSSS is derived through measuring the speed of air passed the metal tube filled with powder, the FSSS value of a powder is associated with the size of particle, meanwhile, it is of related to the strength of the agglomerates. For the as-reduced powders produced by the same sodium reduction process, the lower the FSSS value of the powder is, the larger the surface area is, but for the agglomerated powder, the powders having different surface area may be having similar FSSS value; and for the same capacitance grade powders, the particles with higher agglomeration degree have larger FSSS value. The laser particle size distribution results are derived from that the laser is diffracted and scattered by the sample particles and forms light intensity distribution patterns, depending on the particle size, i.e., outline of the porous or solid particles; and D10, D25, D50, D75 and D90 values are given. The corresponding values of D10, D25, D50, D75 and D90 respectively express the largest particle size of the particles accumulated from the least particle to total of 10 wt %, 25 wt %, 50 wt %, 75 wt % and 90 wt %. The D50 (median) is an indication of the general particle size of a powder. The scanning electron microscopy can be used to observe particle size from micrometer to millimeter, and the transmission electron microscopy can be used to observe nanometer particles.
The BET surface area of the as-reduced tantalum powder produced from sodium reducing K2TaF7 is in the range of 0.2˜6.0 m2/g. The average diameter of the primary particles of tantalum powder of 100000 μF·V/g grade is about 0.1 μm (observed with SEM), the Scott bulk density of the powder is from about 0.4 g/cm3 to about 0.6 g/cm3. The BET surface area of the tantalum powder prepared by reducing Ta2O5 is in the range of from about 1 to about 20 m2/g. The BET surface area of the niobium powder prepared by reducing Nb2O5 is in the range of from about I to about 30 m2/g. The diameter of the primary particle of the powder is in the range of from about 10 to about 350 nm, and the Scott bulk density of the powder is in the range of from about 0.4 g/cm3 to about 0.7 g/cm3.
The tantalum or niobium powder for manufacturing electrolytic capacitors must be having good physical properties, such as suitable Scott bulk density and good flowability. The powder having no flowability is difficult to press into the pellet, and the pellets formed from the metal powders without good flowability have inconsistent weight, result in some troubles, such as large variations of capacitance of the capacitors, heterogeneous density of the pellets and difficult to be coated with the electrolyte and the cathode material, so that the tantalum powder is expected to have a flow rate more than 2.0 g/sec, and the niobium powder is expected to have a flow rate more than 1.0 g/sec.
However, the finer the powder is, the worse the flowability is, so that the fine particles prepared by sodium reduction process have to agglomerate into the porous particles having appropriate Scott bulk density and good flowability.
Moreover, the green pellets pressed from tantalum or niobium powder must be having enough strength. The anodes for electrolytic capacitors sintered from the pellets are expected to have large pore and appropriate distribution of pore size effective to impregnate with solution of manganese nitrate sufficiently, so that pyrolytic manganese dioxide can cover the dielectric tantalum oxide film completely to increase capacitance of the solid electrolytic capacitor, whereas too large pore is disadvantage to decrease the equivalent series resistance (ESR).
Efforts are always being made to improve the physical properties of tantalum powders and niobium powders. WO 99/61184 disclosed a method to agglomerate metal particles and metal particles having improved properties which method includes combining a volatilizable or vaporizable liquid with the particles to form wet particles; compacting the wet particles; drying the compacted wet particles to form a cake; and heat treating the cake to form agglomerated particles. Unfortunately, as the agglomerated particles are angular, the tantalum or niobium powder having CV higher than 80,000 μFV/g made from this method has poor flowability.
Moreover, when vibrating the wet flaked particles (the primary particles are in flaky form, with a aspect ratio (diameter/thickness) 1˜60), an intimate contact tends to form between the surfaces of overlapping flaked particles, thereby decrease the porosity and lower the crush strength of the pellets formed from the powder.
Chinese patent application No. 1238251A disclosed a method for producing agglomerated tantalum powder, but for the powder containing agglomerated particles having D50 more than 50 μm, the agglomerated powder has poor flowability, and can't meet the requirement of manufacture high performance electrolytic capacitors.